A wireless communication system includes wireless personal area networks (WPANs), wireless local area networks (WLANs) and a mobile communication system using code division multiple access (CDMA), global system for mobile communication (GSM), wideband code division multiple access (WCDMA), etc.
Generally, a wireless communication system includes a transmitter for transmitting information (video, voice, data) provided as a radio frequency (RF) signal, and a receiver for receiving the RF signal and obtaining a baseband signal from the received RF signal.
A plurality of filters may be designed for a receiver. Generally, the filters require high filter attenuation characteristics. However, a receiver, which is mounted in a wireless communication system for the local area such as the WPANs and the WLANs, may require low attenuation characteristics of a filter. This is because a receiver of the wireless communication system for the local area has channel spacing relatively broader than a channel bandwidth thereof. Accordingly, a lower order filter may be applied to the receiver of the wireless communication system for the local area. However, to ensure competitiveness of products, requirements for a low power operation are a growing trend.
A transconductance-capacitor (hereinafter, referred to as a Gm-C) filter may be designed in a receiver for the recovery of a received signal and the anti-aliasing of a transmitted signal. The Gm-C filter denotes a filter including a transconductor and a capacitor. The transconductor may mean a circuit outputting an output current proportional to an input voltage. The output current equals a value obtained from the multiplication of the input voltage and the transconductance (gm) of the transconductor. The cut-off frequency of the Gm-C filter may be in proportion to “gm/C”, wherein C is the capacitance of the capacitor.
In a Gm-C filter, a transconductance value (gm value) determining the cut-off frequency varies with a temperature, the variation of a power supply voltage and a manufacturing process. Accordingly, in a case where the Gm-C filter is used, a tuning circuit may be used to constantly keep the cut-off frequency.
A conventional tuning circuit may include a voltage controlled oscillator (VCO) to adjust the transconductance value (gm value) of the transconductor, and thus constantly keeps the cut-off frequency of the Gm-C filter.
However, such a tuning circuit typically requires the designs of a frequency comparator (or a frequency discriminator), a filter (e.g., a low pass filter) included in the inside thereof and wiring for receiving an external clock, in addition to a VCO. Accordingly, it may be difficult to design such a tuning circuit and the size of the resulting tuning circuit may increase, and consequently, such a tuning circuit may consume a high consumption power.